Method of coating core plates and composition therefor



METHOD OF COATING CORE PLATES AND COMPOSITION THEREFOR William J. Frederick and Orlando E. Romig, Pittsburgh, Pa., assignors to United States Steel Corporation, a corporation of New Jersey No Drawing. Application December 7, 1953, Serial No. 396,741

3 Claims. (Cl. 148--6.15)

This invention relates to the production of silicon steel sheet stock for use in the manufacture of electrical equipment and more particularly to the production of such stock possessing high surface resistivity and with a good lamination factor.

Inorganic core plating methods are commonly considered the most economical means for increasing the surface resistivity of the silicon steels and similar magnetic ferrous alloys used in the fabrication of magnetic cores. Known methods, however, are subject to unpredictable variations in result, i. e., the same core plating medium and conditions of application will at times provide surface resistivities of several thousand ohms per :cm?, while at others resistivities below ,5 ohms are produced. This has limited the application of these relatively inexpensive treatments to the lower grades of electrical steels.

It is an object of the present invention to provide an improved inorganic core plating method which is not subject to the aforementioned variations in result and is applicable to all grades of electrical steel stock.

Another object is to provide an improved core plating method capable of consistently producing silicon steel stock having a surface resistivity in excess of 100,000 ohms/cm. /strip at pressures up to 1000 p. s. i. after stress relief or final annealing.

Wider adoption of inorganic core plating has also been prevented by limitations of known coating media, e. g., those media in which sodium silicate is used as the binding agent produce coatings which react with the cooling oils used in large transformers to form undesirable sludges, while those utilizing phosphoric acid produce coatings which are hygroscopic.

Accordingly it is an object to produce an improved inorganic core plating medium free of these deficiencies.

Other objects will be apparent from the following specification.

While the effect of various materials as binding agents in inorganic core plating media has been the subject of considerable study, little attention has been given to the character and nature of the inert dielectric material incorporated into the mixture as the bulking agent. As disclosed in co-pending application of Allen and Romig, Serial No. 396,766, filed December 7, 1953, the particle size of the bulking agent has an important effect on the lamination factor and insulating value of the resulting film. The present invention contemplates utilization of this discovery in combination with the substitution of manganese phosphate or aluminum metaphosphate or mixtures of these two materials for the insoluble siliceous materials commonly used as bulking agents in core plate slurries. We have found that certain quantities of these particular insoluble phosphates dispersed in an aqueous solution of phosphoric acid will produce a nonhygroscopic core plate of exceptionally high insulating power with good lamination factor provided that the particle size of the insoluble phosphates is reduced below about 8 microns and the coating is baked to react the phosphatephosphoric acid mixture with the steel base.

ta St tes A 2,790,739 P en dApr. a0, 1951 In practicing our invention we prepare a slurry of the following ingredients:

Manganese phosphate and/or aluminum metaphosphate gms 300-900 Ortho-phosphoric acid HsPO4) ml 50-110 Water ml 1000 Insofar as our purposes are concerned, manganese phosphate and aluminum metaphosphate are full equivalents on a weight basis. Slurries containing less than 300 gms. total of manganese and/or aluminum phosphate, however, tend to blister, the tendency increasing the higher the phosphoric acid content, while more than 900 gms. total of these materials tends toward dusty coatings, the tendency here being greater at the lower end of the acid range. Various compositions within the foregoing range, however, have shown substantially identical results, i. e., all produce stock characterized by a surface resistivity in excess of 100,000 ohms/cmF/strip at test pressures of 1000 p. s. i. with a good lamination factor, and all the coatings are non-hygroscopic and inactive toward transformer oil.

In preparing the slurries we have found it preferable to mix the ingredients and achieve the necessary degree of fineness by ball-milling the mixture. This produces a more uniform particle size and an unusually stable dispersion of the bulking agents in the acid solution. Ballmilling for about 4 hours is generally sufficient, and with the normally available grades of manganese and aluminum phosphates, reduces the particles thereof to a size between 2 and 5.5 microns.

A core plating medium prepared to the above specifications is applied to the surfaces of the silicon steel stock at any time after the stock has been reduced to final gauge, but preferably just prior to the final annealing thereof for reasons which are readily apparent to those skilled in the art. It is essential that the stock be freed of oil, grease, oxide and other foreign matter before coating. Any method of application which will produce a thin uniform film slurry upon the surfaces of the stock may be used. Immediately following coating, the stock is heated to dry and react the phosphate-phosphoric acid mixture with the iron of the surface of the stock. While this may be accomplished at temperatures as low as 400 F., excessive time is required for complete curing. Curing time is greatly reduced at higher temperatures, however, subjecting the wet coated stock to too high a temperature causes boiling and defective, non-uniform coatings. Accordingly, we prefer to heat the coated stock to a temperature of about 450 F. for sufiicient time to dry the coating and react with the steel base and then raise the temperature to between 1000 and 1500 F. to complete the curing. While time at temperature to achieve complete curing and maximum resistivity varies somewhat with the temperature used, we have found one to two minutes in the aforementioned range is sufficient with the composition of the present method. It is essential, however, that the drying and curing operations as well as any subsequent thermal threatment of the stock be conducted in an oxidizing atmosphere since the electrical resistivity of the core plate is deleteriously affected by heating in reducing atmospheres. While the coated stock is only partially cured after the low temperature drying, the dried coating thereon is very adherent, thus since the temperature range of the curing step includes the temperature normally used in the final annealing of the stock, the curing step may conveniently be combined with this annealing operation, and this procedure constitutes a preferred embodiment of our invention.

It is clear that some modifications may be made in our process without departing from the spirit of our invention; accordingly while we have disclosed certain specific embodiments thereof, We do not wish to be limited exactly thereto except as defined in the appended claims.

We claim: 1. The method of treating ferrous magnetic stock. to

increase the electrical surface resistivity thereof. which comprises applying to cleaned surfaces of the stock a thin film of slurry consisting of 300-900 grams'of an insoluble phosphate selected from the group consisting of manganese phosphate, aluminum metaphosphate and mixtures thereof dispersed in a solution of 50 to 110 mLof 85% phosphoric acid in 1000 ml. of Water, said slurry being characterized by a particle size of said insoluble phosphate ingredient of less than 8 microns, andthen heating the slurry coated stock in an oxidizing atmosphere to a term perature between 400 and 1500 F. to dry and react the phosphoric acid of the slurry with the iron of the stock surfaces.

2. The method of claim 1 in which the slurry coated stock is heated to a temperature of about 450 F. to dry the coating and then subsequently heated to a tempera. ture between 1000 and 1500 F. for sufficient time to completely react the phosphoric acid of the slurry with iron of the stock surfaces.

3. An improved composition for the inorganic coreplating of silicon steel stock consisting of 300-900 grams of an insoluble phosphate selected from the group consisting of manganese phosphate, aluminum metaphosphate and mixtures of manganese phosphate and aluminum metaphosphate, 50 to 1.10 ml. of 85% phosphoric acid and 1000 ml. of water, said composition being characterized by a particle size of the insoluble phosphate ingredient of less than 8 microns.

References Cited in the file of this patent UNITED STATES PATENTS 2,329,065 Lum et a1. Sept. 7, 1943 2,398,529 Holmquist Apr. 16, 1946 2,492,095 Gilford Dec. 20, 1949 2,554,250 Horstman May 22, 1951 2,564,864 Thompson Aug. 21, 1951 FOREIGN PATENTS 638,889 Great Britain June 14, 1950 

1. THE METHOD OF TREATING FERROUS MAGNETIC STOCK TO INCREASE THE ELECTRICAL SURFACE RESISTIVITY THEREOF WHICH COMPRISES APPLYING TO CLEANED SURFACES OF THE STOCK A THIN FILM OF SLURRY CONSISTING OF 300-900 GRAMS OF AN INSOLUBLE PHOSPHATE SELECTED FROM THE GROUP CONSISTING OF MANGANESE PHOSPHATE, ALUMINUM METHAPHOSPHATE AND MIXTURES THEREOF DISPERSED IN A SOLUTION OF 50 TO 110 ML. OF 85% PHOSPHORIC ACID IN 1000 ML. OF WATER, SAID SLURRY BEING CHARACTERIZED BY A PARTICLE SIZE OF SAID INSOLUBLE PHOSPHATE INGREDIENT OF LESS THAN 8 MICRONS, AND THEN HEATING THE SLURRY COATED STOCK IN AN OXIDIZING ATMOSPHERE TO A TEMPERATURE BETWEEN 400 AND 1500*F. TO DRY AND REACT THE PHOSPHORIC ACID OF THE SLURRY WITH THE IRON OF THE STOCK SURFACES. 